refactor: rename GenTx to GenSignedMockTx to avoid confusion with genutil/GenTx

[anilcse]

Description

Closes: #XXXX

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Author Checklist

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I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
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• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
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Reviewers Checklist

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I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[codecov]

Codecov Report

Merging #11696 (bc70a88) into master (1042373) will decrease coverage by 0.02%.
The diff coverage is 94.87%.

@@            Coverage Diff             @@
##           master   #11696      +/-   ##
==========================================
- Coverage   65.92%   65.90%   -0.03%     
==========================================
  Files         667      667              
  Lines       70446    70462      +16     
==========================================
- Hits        46441    46437       -4     
- Misses      21307    21328      +21     
+ Partials     2698     2697       -1     

Impacted Files	Coverage Δ
simapp/test_helpers.go	26.72% <0.00%> (ø)
types/coin.go	93.11% <ø> (ø)
x/genutil/client/cli/gentx.go	0.00% <0.00%> (ø)
x/simulation/util.go	0.00% <0.00%> (ø)
x/authz/simulation/operations.go	66.97% <100.00%> (ø)
x/bank/simulation/operations.go	81.02% <100.00%> (ø)
x/genutil/client/testutil/suite.go	100.00% <100.00%> (ø)
x/gov/simulation/operations.go	85.31% <100.00%> (ø)
x/group/simulation/operations.go	67.73% <100.00%> (ø)
x/nft/simulation/operations.go	69.30% <100.00%> (ø)
... and 3 more